docs: def inv(x):
      '''
      inv(x) yields the inverse of x, 1/x.
  
      Note that inv supports sparse matrices, but it is forced to reify them. Additionally, because
      inv raises an error on divide-by-zero, they are unlikely to work. For better sparse-matrix
      support, see zinv.
      '''
      if sps.issparse(x): return 1.0 / x.toarray()        
      else:               return 1.0 / np.asarray(x)